Embedded fluid mixing device using a homopolar motor

ABSTRACT

A fluid displacement device ( 100 ) having a homopolar motor ( 110 ). The homopolar motor includes a rotatable disk ( 115 ) with at least one fluid displacement structure ( 120 ) disposed thereon. The fluid displacement structure can be a blade. The rotatable disk can be disposed within a cavity ( 145 ) defined in a substrate ( 105 ), such as a ceramic substrate, a liquid crystal polymer substrate, or a semiconductor substrate. A closed loop control circuit ( 235 ) can be included to control the rotational speed of the rotatable disk. For example, the control circuit can control a voltage source or a current source that applies voltage across the rotatable disk. The control circuit also can control a strength of a magnet ( 210 ) that applies a magnetic field ( 205 ) substantially aligned with an axis or rotation ( 155 ) of the rotatable disk.

BACKGROUND OF THE INVENTION

1. Statement of the Technical Field

The inventive arrangements relate generally to the field of microelectromechanical system (MEMS) devices.

2. Description of the Related Art

As technology progresses, clock speeds of modern electronic devicescontinue to increase, resulting in a greater amount of heat beingproduced by device components. Additionally, efforts are currently underway to adapt very small fuel cells, called microcells, into portableelectronic devices. For example, it is anticipated that microcells soonwill be used for powering laptop computers and cell phones. Microcellsgenerate heat, though, thus adding to the total amount of heat that willbe generated by such electronic devices. Meanwhile, circuit geometriesand device packaging continue to shrink. Hence, modern circuit designersface many design challenges concerning thermal management.

Fans are a frequently used solution to dissipate heat within electronicdevices. However, such fans tend to be rather bulky, occupying valuablespace within the devices. Moreover, the fans often prove to be less thanreliable, sometimes failing prior to any other device components. Insome instances, the loss of heat dissipation resulting from the failureof a fan actually causes other device components to overheat and fail.Accordingly, a small, yet reliable thermal management solution is neededto dissipate heat in modern electronic devices.

SUMMARY OF THE INVENTION

The present invention relates to a fluid displacement device having ahomopolar motor. The homopolar motor includes a rotatable disk with atleast one fluid displacement structure disposed thereon, for example ablade. The rotatable disk can be at least partially disposed within acavity defined in a substrate, such as a ceramic substrate, a liquidcrystal polymer substrate, or a semiconductor substrate. A closed loopcontrol circuit can be included to control the rotational speed of therotatable disk. For example, the control circuit can control a voltagesource or a current source that applies voltage across the rotatabledisk. The control circuit also can control a strength of a magnet thatapplies a magnetic field substantially aligned with an axis or rotationof the rotatable disk.

Voltage can be applied across a central portion and a radial edgeportion of the rotatable disk in the presence of a magnetic fieldsubstantially aligned with an axis of rotation of the rotatable disk.The rotatable disk can have at least one fluid displacement memberdisposed thereon, for example a blade. The rotational speed of therotatable disk can be selectively controlled to vary a fluiddisplacement rate. For example, the control circuit can control avoltage source or a current source that applies voltage across therotatable disk. The control circuit also can control the strength of themagnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a fluid mixer that is useful forunderstanding the present invention.

FIG. 2 is a section view of the fluid mixer of FIG. 1 taken alongsection line 2-2.

FIGS. 3A-3C illustrate a process for manufacturing the fluid mixer on adielectric substrate, which is useful for understanding the presentinvention.

FIGS. 4A-4I illustrate a process for manufacturing the fluid mixer on asemiconductor substrate, which is useful for understanding the presentinvention.

FIG. 5 is a flow chart that is useful for understanding the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a fluid displacement device (fluidmixer) embedded within a substrate. Accordingly, the fluid mixer can bemanufactured as a micro electromechanical system (MEMS) device. Thefluid mixer can be embedded proximate to thermal generating devices andused to enhance the flow of convection currents around the devices,thereby providing a low profile solution for improving device heatdissipation. Accordingly, the use of bulky cooling fans which otherwisemight be needed can be avoided. In another arrangement, the fluid mixercan be embedded within a substrate of a microfluidic system to mixfluids. Thus, the use of external mixing devices can be avoided.

The fluid mixer can be a stand alone device or can be advantageouslyintegrated within a larger system. Examples of such larger systems caninclude electronic devices, fuel cells, fluidic systems, or any otherdevice having a substrate. Importantly, the invention is not limited toany particular type of device.

Referring to FIG. 1, a perspective view of a fluid mixer 100 inaccordance with the present invention is shown. The fluid mixer 100 canbe manufactured on a substrate 105, which can be any of a variety ofsubstrates. For example, the fluid mixer 100 can be manufactured on asubstrate made of liquid crystal polymer (LCP), ceramic, silicon,gallium arsenide, gallium nitride, germanium or indium phosphide. Still,the invention is not so limited and any substrate material suitable fora microelectromechanical manufacturing process can be used.

The fluid mixer 100 can include a microelectromechanical homopolar motor(homopolar motor) 110 having a conductive rotatable disk (disk) 115. Atleast one fluid displacement structure can be disposed on the disk 115.The fluid displacement structure can be any structure that can be movedby the disk 115 and that is suitable for displacing fluids. For example,fluid displacement structures (blades) 120 can be disposed on therotatable disk 115 such that the blades 120 mix fluid as the disk 115rotates. For instance, the blades 120 can extend upwards from an uppersurface 125 of the disk 115. The blades 120 can be integrally formed onthe disk 115 or attached to the disk via glue, fasteners, a weld, or anyother suitable attachment means. The blades 120 can extend from acentral portion 130 of the disk 115 to an outer peripheral region 135 ofthe disk 115. In one arrangement the blades 120 can extend radially fromthe central portion 130 of the disk 115 in a linear fashion. However,the invention is not so limited. For example, the blades 120 can becurved, angled, or have any other desired shape. Moreover, fluiddisplacement structures having complex mechanical configurations can beprovided. For instance, the blades 120 can include a plurality of curvedand/or angled portions configured to optimize fluid displacement in theapplication for which the fluid mixer 100 will be used.

In addition to the blades 120, or in lieu of the blades 120, other typesof fluid displacement structures also can be provided. For instance, thefluid displacement structures can comprise a plurality of corrugationsand/or raised nubs disposed on the upper surface 125 of the disk 115.Still, there are many other types of fluid displacement structures thatcan be used and the invention is not so limited.

The disk 115 can be positioned proximate to a surface 140 of thesubstrate 105, for example within a cavity 145 defined within asubstrate 105. Importantly, the cavity 145 can have a shape that issubstantially circular, square, rectangular, or any other desired shape.Nevertheless, it should be noted that a cavity is not required topractice the present invention. For instance, the disk 115 can bedisposed above the surface 140 of the substrate 105.

In one arrangement, the disk 115 can be provided with an axle 150 tofacilitate rotation about the central axis 155 of the disk 115 andmaintain the disk 115 in the proper operating position. Nevertheless,other arrangements can be provided as well. For example, in anotherarrangement the cavity 145 can be structured with a low frictionperipheral surface 160 that maintains the disk 115 within the cavity145. In yet another arrangement, a hole can be provided at the centralaxis 155 of the disk 115. The hole can fit over a cylindrical structure,such as a bearing, to maintain the operating position of the disk 115.

In operation, rotation of the disk 115 rotates the fluid displacementstructure, such as blades 120, about the central axis 155, moving theblades 120 through a fluid medium. Accordingly, the blades 120 can causefluid to be displaced. For example, movement of the blades 120 canpromote mixing of two or more fluids. Movement of the blades 120 alsocan disrupt boundary layer fluid flow over a surface, thereby enhancingconvection heat transfer.

Referring to FIG. 2, a cross section is shown of the fluid mixer 100 ofFIG. 1 taken along section line 2-2. The rotatable disk 115 is immersedin a magnetic field, illustrated with magnetic field lines 205, whichare typically perpendicular to the surface 125 of the disk 115 andaligned with the axis of rotation 155 of the disk. One or more magnets210 can be provided above and/or below the disk 115 to generate themagnetic field 205. The magnets 210 can include permanent magnets and/orelectromagnets.

A first contact brush 215 can contact the disk 115 near its centralportion 130, which is proximate to the disk central axis 155. A secondcontact brush 220, which can be radially spaced from the first contactbrush 215 to contact the radial edge portion 225 of the disk 115. Thesecond contact brush 220 can contact the radial edge portion 225 at asingle point, or circumferentially extend under or around the entireradial edge portion 225.

In one arrangement, a contact brush (not shown) can be provided tocontact the axle 150. Additional contact brushes also can be provided.For example, contact brushes can be spaced in a circular pattern tocontact multiple points on the radial edge portion 225. Similarly,contact brushes can be spaced near the central portion 130 of the disk115 to contact the central portion 130 at multiple points, to form acontinuous circumferential contact surface at the central portion 130,or to contact the axle 150.

When voltage is applied across the contact brushes 215 and 220, causingcurrent to flow through the disk 115, magnetic forces are exerted on themoving charges. The moving charges in turn exert the force to the disk115, thereby causing the disk 115 to rotate. Notably, the direction ofrotation depends on the direction of the current flow through the disk115, for example, whether the current flows from the central portion 130of the disk 115 to the radial edge portion 225, and vice versa.Accordingly, the polarity of the applied voltage can be changed when itis desired to change the direction of rotation of the conducive disk115.

Further, a sensor 230 can be provided for monitoring the rotationalspeed of the disk 115. For instance, the sensor 230 can be operativelyconnected to a closed loop control system 235 which controls therotational speed. Such sensors are known to the skilled artisan. Forexample, the sensor can be an optical sensor which reads one or moremarks on the disk 115 as the disk 115 rotates. In another arrangement,the sensor can generate a signal each time a blade passes the sensor asthe disk rotates. The time period between sequential mark readings (orblades passing the sensor) can be measured and correlated to therotational speed of the disk 115. Still, there are a myriad of othersensors known to the skilled artisan that can be used to measure orderive the rotational speed of the disk, and the invention is not solimited.

Regardless of how rotational speed is determined, the control system 235can control the rotational speed of the disk 115 by controlling avoltage and/or current source 240 that applies voltage to the disk 115.The control system 235 also can control the rotational speed bycontrolling the field strength 205 of the magnets 210. For instance, inthe case that the magnet 210 comprises an electromagnet, electriccurrent through the electromagnet can be adjusted.

FIGS. 3A-3C represent one manufacturing process that can be used formanufacturing the fluid mixer on a ceramic substrate. Nevertheless, itshould be noted that the structures represented in FIGS. 3A-3C also canbe implemented for manufacturing the fluid mixer with other types ofsubstrates, for example with LCP substrates. It should be noted,however, that the lamination and curing processes can differ for eachtype of substrate, as would be known to the skilled artisan.

One LCP substrate that can be used is R/flex® 3000 Series LCP CircuitMaterial available from Rogers Corporation of Rogers, Conn. The R/flex®3000 LCP has a low loss tangent and low moisture absorption, andmaintains stable electrical, mechanical and dimensional properties. TheR/flex® 3000 LCP is available in a standard thickness of 50 μm, but canbe provided in other thicknesses as well.

One ceramic substrate that can be used is low temperature 951 co-fireGreen Tape™ from Dupont®. The 951 co-fire Green Tape™ is Au and Agcompatible, and has acceptable mechanical properties with regard tothermal coefficient of expansion (TCE) and relative strength. It isavailable in thicknesses ranging from 114 μm to 254 μm. Other similartypes of systems include a material known as CT2000 from W. C. HeraeusGmbH, and A6S type LTCC from Ferro Electronic Materials of Vista, Calif.Any of these materials, as well as a variety of other LTCC materialswith varying electrical properties can be used.

Referring to FIG. 3A, a first substrate layer 302 can be provided. Thesubstrate material that is to be used in each of the substrate layerscan be preconditioned before being used in a fabrication process. Forexample, if the substrate is ceramic, the ceramic material can be bakedat an appropriate temperature for a specified period of time or left tostand in a nitrogen dry box for a specified period of time. Commonpreconditioning cycles are 160° C. for 20-30 minutes or 24 hours in anitrogen dry box. Both preconditioning process are well known in the artof ceramic substrates.

Once the first substrate layer 302 is preconditioned, a conductive via320 can be formed in the first substrate layer 302 to provide electricalconductivity through the substrate layer. Many techniques are availablefor forming conductive vias in a substrate. For example, vias can beformed by mechanically punching holes or laser cutting holes into thesubstrate. The holes then can be filled with a conductive material, suchas a conventional thick film screen printer or extrusion via filler.Vacuum can be applied to the first substrate layer 302 through a porousstone to aid via filling. Once the conductive via 320 has been formed inthe first substrate layer 302, the conductive material can be dried in abox oven at an appropriate temperature and for an appropriate amount oftime. For example, a common drying process is to bake the ceramicsubstrate having the conductive material at 160° C. for 5 minutes.

After the conductive filler in the via has dried, a first conductivecircuit trace 325 and a second conductive circuit trace 330 can beprovided. The circuit traces 325, 330 can be deposited onto the firstsubstrate layer 302 using a conventional thick film screen printer, forexample, standard emulsion thick film screens. In one arrangement, thecircuit traces 325, 330 can be deposited onto opposite sides of thefirst substrate layer 302, with the first circuit trace 325 being inelectrical contact with the conductive via 320. The second circuit trace330 can extend around, and be concentric with, the conductive via 320.Nonetheless, a myriad of other circuit layouts can be provided, as wouldbe known to the skilled artisan. As with the via filling process, oncethe circuit traces have been applied to the first substrate layer 302,the circuit traces can be dried in a box oven at an appropriatetemperature and for an appropriate amount of time.

Subsequent substrate layers can be laminated to the first substratelayer 302 after appropriate preconditioning and drying of the circuittraces and/or via fillers. In particular, a second substrate layer 304can be stacked onto the first substrate layer 302. The second layer 304can insulate circuit traces on the top of the first substrate layer 302.The second substrate layer also can include vias 335, 340, which can befilled with material to form an axial contact brush 345 and at least oneradial contact brush 350, respectively. The vias can be positioned sothat the contact brushes are electrically continuous with respectivecircuit traces 325, 330. In one arrangement, a plurality of radialcontact brushes 350 or a continuous radial edge contact brush can bedisposed concentric with, and at a uniform radius from, the axialcontact brush 345 to reduce a net contact resistance between theconductive object and the brushes.

The contact brushes can include any conductive material suitable for usein a contact brush, for example a conductive epoxy, conductive polymer,carbon nano composite or a conductive liquid. In the case that thecontact brushes are a solid material, such as carbon nano composite, thecontact brushes can be screen printed into the vias in the secondsubstrate layer 304 using a conventional thick film screen printer. Inthe case that a conductive liquid is used as contact brushes,ferromagnetic properties can be incorporated into the conductive liquidso that a magnetic field can contain the conductive liquid within thevias 335, 340. In one arrangement, the axial contact brush 345 can fillonly part of the via 335 so that a top surface of the contact brush 345is disposed below an upper surface 355 of the second substrate layer304. Accordingly, the via 335 also can function as a bearing.

A third substrate layer 306 can be stacked above the second substratelayer 304. The third substrate layer 306 can incorporate an aperture 360having a radius edge 365 aligned with an outer radius of vias 350 (aportion of each via furthest from the via 335). A fourth substrate layer308 can be stacked below the first substrate layer 302 to insulatecircuit traces on the lower surface 370 of the first substrate layer302. Further, a fifth substrate layer 310 can be stacked below thefourth substrate layer 308. The fifth substrate layer 310 also caninclude an aperture 375 having an outer radius 380.

In some instances it can also be desirable to include a conductiveground plane (not shown) on at least one side of one or more of thesubstrate layers 302, 304, 306, 308, 310. For example, the ground planecan be used in those instances where RF circuitry is formed on thesurface of a substrate layer. The conductive ground plane also can beused for shielding components from exposure to RF and for a wide varietyof other purposes. The conductive metal ground plane can be formed of aconductive metal that is compatible with the substrate. Still, thoseskilled in the art will appreciate that the ground plane is not requiredfor the purposes of the invention.

Referring to FIG. 3B, the first five layers 302, 304, 306, 308, 310 canbe stacked to form a substrate structure 385. Importantly, it should benoted that the layer scheme presented herein is by example only. Agreater number or a fewer number of substrate layers also can be used.Notably, each of the substrate layers can further comprise multiple sublayers which have been stacked to form each layer.

Once the substrate layers have been stacked to form the substratestructure 385, the structure 385 can be laminated using a variety oflamination methods. In one method, the substrate layers can be stackedand hydraulically pressed with heated platens. For example, a uniaxiallamination method presses the substrate layers together at 3000 psi for10 minutes using plates heated to 70° C. The substrate layers can berotated 165° following the first 5 minutes. In an isotatic laminationprocess, the substrate layers are vacuum sealed in a plastic bag andthen pressed using heated water. The time, temperature and pressure canbe the same as those used in the uniaxial lamination process; however,rotation after 5 minutes is not required. Once laminated, the structure385 can be fired inside a kiln on a flat tile. For example, thesubstrate layers can be baked between 200° C. and 500° C. for one hourand a peak temperature between 850° and 875° can be applied for greaterthan 15 minutes. After the firing process, post fire operations can beperformed on the substrate layers.

Referring to FIG. 3C, the disk 115 can be provided within the cavity145, formed by aperture 360. The disk 115 can comprise a conductivematerial, such as aluminum, copper, brass, silver, gold, steel,stainless steel, or any other rigid conductive material. In anotherarrangement, the disk 115 can comprise a plurality of materials, forexample a semi-rigid conductive material that is laminated to a rigidmaterial, for instance ceramic. The disk 115 can include a centralcontact 390 axially located on the lower surface 392, and at least oneradial contact 395, also located on the lower surface 392. In onearrangement, the radial contact 395 can extend around the lowerperipheral region 397 of the disk 115. The disk 115 can be positionedabove the second substrate layer 304 so that the central contact 390makes electrical contact with the axial contact brush 345 and the radialcontact 395 makes electrical contact with the radial edge contact brush350. Accordingly, electrical current can flow between the centralportion 130 of the disk and radial edge portion 225 when voltage isapplied across the contact brushes 345, 350. A radial wall 398 of thevia 335 can function as a bearing surface for the central contact 390 ofthe disk 115. Alternatively, bearings (not shown) can be installedbetween the radial wall 398 and the central contact 390. The bearingscan be, for example, electromagnetic or electrostatic bearings.

As noted, a sensor 230 can be provided for use in a control circuit forcontrolling operation of the disk 115. The sensor 230 can be disposed inany location suitable for measuring rotational speed of the disk 115.Circuit traces can be provided as required for propagating sensor data,as would be known to the skilled artisan.

One or more magnets can be fixed above and/or below the disk 115 toprovide the magnetic field aligned with an axis of rotation of the disk115. For example, a magnet 210 can be attached to the bottom of thesubstrate structure 385, for example in the aperture 375, such that themagnet 210 is spaced from the lower surface 392 of the disk 115.Nonetheless, the invention is not limited in this regard. For instance,a magnet 210 also can be spaced from the upper surface 125 of the disk115. The magnet 210 can be a permanent magnet, such as a magnet formedof magnetic material. For example, the magnet 210 can be made offerrite, neodymium, alnico, ceramic, and/or any other material that canbe used to generate a magnetic field.

The magnet 210 also can be a non-permanent magnet, for example, anelectromagnet. In another arrangement, the magnet can be a combinationof one or more permanent magnets and one or more non-permanent magnets,for example, an electromagnet adjacent to one or more layers of magneticmaterial. As previously noted, the strength of the magnetic fieldgenerated by an electromagnet can be varied by varying the currentthrough the conductor of the electromagnet, which can provide anadditional means for controlling the amount of rotation of the disk 115.

In another exemplary embodiment, the fluid mixer 100 can be manufacturedon a semiconductor substrate, for example on a silicon substrate using apolysilicon microfabrication process. Polysilicon microfabrication iswell known in the art of micromachining. One such process is disclosedin David A. Koester et al., MUMPs Design Handbook (Rev. 7.0, 2001). Anexemplary polysilicon microfabrication process is shown in FIGS. 4A-4I.It should be noted, however, that the invention is not limited to theprocess disclosed herein and that other semiconductor microfabricationprocesses can be used.

Referring to FIG. 4A, a first silicon substrate layer (first siliconlayer) 402 can be provided to begin forming the fluid mixer structure400, for example, a silicon wafer typically used in IC manufacturing. Insome cases, it may be desirable for the first silicon layer 402 to haveelectrically insulating properties. Accordingly, the first silicon layer402 can be formed without doping or have only a light doping.

A conductive layer, for example a layer of doped polysilicon oraluminum, can be deposited onto the first substrate layer 402. Afterdeposition of the conductive layer, conductive circuit traces 404 can bedefined using known lithography and etching techniques. In some cases itcan be advantageous to deposit an insulating layer (not shown) such assilicon nitride (SiN) over the first silicon layer 402 prior todepositing the conductive layer used to form the conductive traces 404.

After the circuit traces are formed, an insulating layer 406 can bedeposited onto the first silicon layer 402 using low pressure chemicalvapor deposition (LPCVD). Inner vias 408, and outer vias 410 then can beformed in the insulating layer 406, for example using etchingtechniques. In the case that an insulating layer is provided over thecircuit traces, the insulating layer also should be etched at thelocation of the vias 408, 410.

The inner vias 408 and outer vias 410 can be filled with electricallyconductive material (e.g. aluminum) to electrically contact the circuittraces 404 at desired locations. Axial contact brushes 412 then can bedeposited on inner vias 408 and radial edge contact brushes 414 can bedeposited on outer vias 410 so that the contact brushes 412 and 414 areelectrically continuous with the respective vias 408 and 410 andcorrelating circuit traces 404. Two axial contact brushes 412 and tworadial edge contact brushes 414 are shown in the figure, but additionalaxial and radial edge contact brushes can be provided. Further, thecontact brushes can include any conductive material suitable for use ina contact brush, for example, a carbon nano composite which can beapplied using a thermo spray method commonly known to the skilledartisan. In another arrangement, the contact brushes can be a conductiveliquid.

A first structural layer of polysilicon (poly 1 layer) 416 can bedeposited onto the insulating layer 406 using LPCVD. The poly 1 layer416 then can be etched to form a radial aperture 418, which exposes thecontact brushes 412, 414. In an alternate arrangement, the aperture 418region can be masked prior to application of the poly 1 layer 416,thereby preventing deposition in the aperture 418 region.

Referring to FIG. 4B, a first sacrificial layer 420, for example silicondioxide (SiO₂) or phosphosilicate glass (PSG), can be applied to thesubstrate over the previously applied layers. The first sacrificiallayer 420 can be removed at the end of the process. The sacrificiallayer can be deposited by LPCVD and annealed to the circuit. Referringto FIG. 4C, the first sacrificial layer 420 then can be planarizedwithin the aperture 418 using a planarizing etch-back process to form aflat base 422 within the aperture 418 that is recessed from an upperelevation 424 of the first sacrificial layer 420.

Referring to FIG. 4D, a conductor then can be deposited into theaperture 418 to form a disk (disk) 426 having opposing upper surface428, a lower surface 430, an axial portion 432, and a radial edgeportion 434. Further, the disk 426 can be wholly contained within theaperture 418 so that the only material contacting the disk 426 is thefirst sacrificial layer 420. The thickness of the disk 426 can bedetermined by the thickness of the first sacrificial layer 420 and theamount of etch-back. Importantly, mechanical characteristics, such asrigidity, should be considered when selecting a thickness for the disk426.

Referring to FIG. 4E, a second sacrificial layer 436 can be applied tothe substrate over the previously applied layers. Again, the secondsacrificial layer 436 can be deposited by LPCVD and annealed to thecircuit and can be removed at the end of the process. Blade portions 438of the sacrificial layer 436 can be etched away to expose regions of thedisk 426 extending radially from the axial portion 432 of the disk 426to the radial edge portion 434 of the disk 426. A conductor then can bedeposited into the blade regions 438 to form blades 440 that are affixedto the disk 426, as shown in FIG. 4F.

Referring to FIG. 4G, an orifice 442 then can be etched through theinner region of the disk 426 and through the first and secondsacrificial layers 420, 436, thereby exposing a region 448 of theinsulating layer 406 below the center of the disk 426. Known etchingtechniques can be used, for example reactive ion etch (RIE), plasmaetching, etc. Notably, the first orifice 442 can be sized to form a holein the disk 426 having a radius equal to or smaller than the radialdistance between opposing axial contact brushes 412, 414.

A third sacrificial layer 444 then can be applied over the disk 426, theblades 440 and over the radial wall 446 formed by the orifice 442. Theregion 448 of the insulating layer 406 should be masked during theapplication of the third sacrificial layer 444 to prevent the thirdsacrificial layer 444 from adhering to the insulating layer 406 in theregion 448. Alternatively, a subsequent etching process can be performedto clear away the third sacrificial layer from the region 448.

Referring to FIG. 4H, using LPCVD, a second layer of polysilcon (poly 2layer) 450 can be deposited over the previously applied layers, forexample over the third sacrificial layer 444, thereby adding anadditional silicon structure. Notably, the poly 2 layer 450 also canfill the orifice 442. Notably, the poly 2 layer 450 can be formed tohave an outer radius 452 that is larger than an inner radius 454 of thedisk 426. Accordingly, the poly 2 layer 450 can be formed to have a “T”shaped cross section extending upward from the region 448 of theinsulating layer 406, thereby limiting vertical movement of the disk 426once the sacrificial layers are removed. Further, the poly 2 layer canoperate as a bearing around which the disk 426 can rotate.Alternatively, electromagnetic or electrostatic bearings can be providedin the first orifice 442.

The first, second and third sacrificial layers 420, 436, 444 then can bereleased from the fluid mixer structure 400, for example using ahydrogen fluoride (HF) solution. Such a process is known to the skilledartisan. For example, the fluid mixer structure 400 can be dipped in anHF bath. HF does not attack silicon or polysilicon, but quickly etchesSiO₂. Notably, the HF can etch deposited SiO₂ approximately 100× fasterthan SiN.

Referring to FIG. 4I, the release of the sacrificial layers enables thelower portion 430 of the disk 426 to rest upon, and make electricalcontact with, the axial and radial edge contact brushes 412, 414. Thedisk 426, along with blades 440, then can be free to rotate about theaxis of the disk. In one arrangement, a gasket 456 can be disposedbetween the T-shaped poly 2 layer 450 and the disk 426 to maintain theposition of the disk 426 in contact with contact brushes 412, 414. Forexample, the gasket 456 can comprise a photodefinable polymer, such as abenzocyclobutene-based polymer, polyimide or SU-8. Such polymers arecommercially available. For instance, SU-8 is commercially availablefrom MicroChem Inc. of Newton, Mass. 02164. Teflon and Vespel, availablefrom Dupont®, also are materials that can be used for the gasket 456.

In another arrangement, a framework with standoffs can be attached tothe insulating layer 406 and/or the poly 1 layer 416. The standoffs canto maintain the position of the disk 426 in contact with contact brushes412, 414. The standoffs can comprise a photodefinable polymer, Teflon,or Vespel. Additionally, the framework can be perforated to allow fluidflow. Alternatively, aerodynamic forces caused by rotation of the disk426 and blades 440 can maintain the position of the disk 426 in contactwith contact brushes 412, 414.

A magnet 460 can be fixed above and/or below the disk 426 to provide amagnetic field aligned with the axis of rotation of the disk 426. Forexample, the magnet 460 can be attached to the first substrate layer 402below the disk 426. As previously noted, the magnet can be a permanentmagnet, non-permanent magnets, or a combination of a permanent magnetand a non-permanent magnet. Also as noted, a sensor 458 also can beprovided for monitoring rotational speed of the disk.

A flow chart 500 which is useful for understanding the method of thepresent invention is shown in FIG. 5. Beginning at step 505, a cavitycan be formed within the substrate. Proceeding to step 510, contactbrushes can be formed on the substrate within the cavity. At least onecontact brush can be disposed proximate to a central portion of thecavity and at least one contact brush can be disposed proximate to aradial edge portion of the cavity. Continuing at step 515, a conductivedisk having an axial portion and a radial edge portion then can bedisposed within the cavity. The conductive disk can have fluiddisplacement structures disposed thereon. The conductive disk can bedisposed to make electrical contact with the contact brushes. Referringto step 520, a magnet can be disposed on the substrate to define amagnetic field aligned with an axis of rotation of the conductive disk.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as described in theclaims.

1. A fluid displacement device comprising: a homopolar motor comprisinga rotatable disk; and at least one fluid displacement structure providedon said rotatable disk.
 2. The fluid displacement device of claim 1wherein said rotatable disk is disposed on a substrate.
 3. The fluiddisplacement device of claim 2 wherein said rotatable disk is at leastpartially disposed within a cavity defined in said substrate.
 4. Thefluid displacement device of claim 2 wherein said substrate is selectedfrom the group consisting of a ceramic substrate, a liquid crystalpolymer substrate, and a semiconductor substrate.
 5. The fluiddisplacement device of claim 1 further comprising a closed loop controlcircuit to control a rotational speed of said rotatable disk.
 6. Thefluid displacement device of claim 5 wherein said closed loop controlcircuit controls at least one of a voltage source and a current sourcethat apply voltage across said rotatable disk.
 7. The fluid displacementdevice of claim 5 wherein said closed loop control circuit controls astrength of a magnet that applies a magnetic field substantially alignedwith an axis of rotation of said rotatable disk.
 8. The fluiddisplacement device of claim 1 wherein said at least one fluiddisplacement structure comprises a blade.
 9. A method for displacingfluid comprising: positioning within a fluid a rotable disk having atleast one fluid displacement structure disposed thereon; and applying avoltage across a central portion of the rotatable disk and a radial edgeportion of the rotatable disk in the presence of a magnetic fieldsubstantially aligned with an axis of rotation of the rotatable disk.10. The method according to claim 9 further comprising selectivelycontrolling a rotational speed of the rotatable disk to vary a fluiddisplacement rate.
 11. The method according to claim 10 wherein saidstep of selectively controlling a rotational speed further comprisescontrolling at least one of a voltage source and a current sourceapplying the voltage across the rotatable disk.
 12. The method accordingto claim 10 wherein said step of selectively controlling a rotationalspeed further comprises controlling a strength of the magnetic field.13. The method according to claim 9 further comprising selecting a bladeto be the fluid displacement member.